P0420 on a 2019 Toyota Camry
P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.
Not your vehicle? See general P0420 info.
Most common causes on the 2019 Toyota Camry
Ranked from 385 NHTSA owner complaints filed for this vehicle.
- 1
Cylinder misfire fouling/damaging the catalyst
High severityOne owner's diagnostic pulled a P0302 cylinder misfire alongside rough running, shaking, and reduced power — unburned fuel from a misfire is a classic driver of catalyst damage and P0420.
Reported in 1 NHTSA complaint - 2
Head gasket failure allowing coolant/oil into the exhaust and contaminating the catalyst
High severityOne owner needed a full engine replacement after a blown head gasket around 116k miles; coolant or oil entering the exhaust stream is a recognized cause of catalyst contamination and efficiency codes.
Reported in 1 NHTSA complaint - 3
Fuel system leak causing a rich condition that fouls the catalyst
High severityOne owner reports a fuel line coming loose while driving, pushing the engine into limp mode; raw fuel or an uncontrolled rich condition can quickly foul a catalyst.
Reported in 1 NHTSA complaintParts:fuel line - 4
Worn/degraded catalytic converter substrate (normal aging)
Medium severityAge and mileage reduce O2 storage capacity below the ECM's efficiency threshold.
Common causeParts:catalytic converter - 5
Failing downstream oxygen sensor
Medium severityA biased or slow rear O2 sensor can make a healthy converter appear inefficient.
Common causeParts:oxygen sensor - 6
Exhaust leak ahead of the sensor
Medium severityA leaking manifold gasket or pipe joint distorts the efficiency comparison.
Common causeParts:exhaust manifold gasket
Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.
These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.
Common symptoms
- Check engine light
- Usually no drivability issue
- Failed emissions test
- Sulfur smell in some cases
- Reduced power if converter is clogged
What to check before replacing parts
Check for misfire and fuel trim codes first
Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.
Inspect for exhaust leaks ahead of the converter
Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.
Compare upstream and downstream O2 sensor activity
Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.
Check converter temperature and performance
Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.
Hold off on replacing the converter until root causes are ruled out
Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.
Rather have a pro handle it? Find a vetted local mechanic.